The present invention relates to a drive device for a single-phase series commutator motor.
(Outline of a Single-Phase Series Commutator Motor)
A single-phase series commutator motor can be rotary-driven with either of a direct current (DC) power source and an alternating current (AC) power source, and therefore is called a universal motor. This kind of motor can rotate at a high speed, receives minimal influence from power supply frequency, and can be produced at low cost. Therefore, this kind of motor is widely used for an electric power tool powered by an AC power source.
The motor includes an armature winding and a field winding serially connected to each other. The motor is configured to be rotary-driven by a torque generated when a power source is serially connected to the armature winding and the field winding.
In the motor, regardless of polarity of the AC power source or the DC power source, a direction of the generated torque is fixed constantly to one direction unless a connection configuration of the armature winding and the field winding is changed.
By reversing a connecting direction of either one of the armature winding and the field winding, a switching of the direction of the generated torque is achieved.
Accordingly, an application in which the direction of the torque generated in the motor is switched can be achieved by providing single pole double throw switches at both ends of the field winding in order to switch the connecting direction.
(Braking of the Motor)
An example of the application in which the direction of the generated torque is switched is a braking required to stop the motor rapidly.
In this example, the braking of the motor is achieved by generating a counter torque in the motor that is being rotary-driven by the torque in one direction.
In a normal case, the armature winding and the field winding serially connected to each other are connected to a power source. By reversing a connecting direction of either one of the armature winding and the field winding, a current loop is formed in which both ends of these serially connected windings are connected with/without the resistor interposed therebetween. As a result, a torque in a direction reverse to that in the normal case is generated and the motor is thereby braked.
(Problems with the Above-Described Braking)
In performing the braking as above, a braking current corresponding to the number of rotations of the motor flows through the current loop during a period when the motor is braked. Therefore, an excessive current may flow at the beginning of the braking, and thus, the commutator of the motor disposed within the current loop may deteriorate due to the excessive current.
(Case Example of a Braking Current and a Constant Current Control)
In order to suppress the above-described problems, a device disclosed, for example, in Japanese Patent Publication No. 2,735,771 is configured as follows.
The device is configured to detect a braking current flowing through a braking current path formed of an armature and a field winding during a period when the motor is braked; to interrupt the braking current path when the braking current reaches a predetermined upper limit; and to connect both ends of the field winding via a resistor for consuming a current.
In the device configured as such, when the braking current reaches the predetermined upper limit, the both ends of the field winding are connected via the resistor for consuming current, and a circulating current flows through the field winding via the resistor. As a result, energy stored in the field winding is consumed by the resistor.
Then, in the device, when the circulating current flowing through the field winding via the resistor is decreased, the armature and the field winding form the braking current path again.
Consequently, according to the device, it is possible to stop the motor by generating a braking torque in the motor while the braking current is suppressed to or below the upper limit during the period when the motor is braked.